1. Field of the Invention
The present invention relates to granular titanate ion exchangers and to a method of producing such ion exchangers.
2. Description of Related Art
There are many objectives for the separation of radionuclides from nuclear waste solutions, e.g., minimization of volumes of final wastes, declassification or exemption of large amounts of wastes, and minimization of radioactive releases into the environment. The radionuclides of cesium, .sup.134,137 Cs, are present in most waste solutions and these are often responsible for most of the total radioactivity contained in waste. However, in waste solutions of nuclear fuel reprocessing and nuclear weapons manufacturing plants radioactive strontium, .sup.90 Sr, is just as much of a problem as cesium; both are fission products, which have rather long half-lifes (30 years) and high fission yields.
A major waste stream arising from above-mentioned plants comprises the alkaline concentrated salt solutions. In these solutions radioactive strontium, .sup.90 Sr, and cesium make up most of the total activity of solutions, whereas the majority of the other radionuclides are precipitated onto the container bottom.
For the removal of soluble radionuclides both precipitation and ion exchange methods have been used. Ion exchange is munch more straightforward and provides better decontamination and volume reduction.
Organic ion exchange resins are being extensively used in the nuclear power industry for the removal of radionuclides from solutions, especially in the purification of primary coolant and low-salt waste solutions at nuclear power plants. Unlike many inorganic ion exchangers, organic resins are not, however, very ion selective and they are not resistant to high temperatures and radiation doses. As a result, for highly concentrated salt solutions and highly active waste solutions, selective inorganic ion exchangers are the only choice, as described in Lehto, J., Ion Exchange in the Nuclear Power Industry, in: Ion Exchange Processes: Advances and Applications, Proceedings of ION-EX'93, Royal Society of Chemistry, 1993, p. 39.
An efficient hexacyano ferrate-based ion exchange material has been developed for the selective separation of cesium from a wide variety of nuclear waste solutions. Said material has been used on an industrial-scale since 1991 (cf. Harjula, R., Lehto, J., Tusa, E., and Paavola, A., Industrial Scale Removal of Cesium with Hexayanoferrate Exchangers--Process Development, Nucl. Technol. 107 (1994) 272).
For strontium, titanates and hydrous titanium oxides have been shown to be effective as ion exchangers.
Crystalline sodium titanates belong to either of the following series Na.sub.2 Ti.sub.n O.sub.2n-1 (members with N=1-9 have been reported) and Na.sub.4 Ti.sub.n O.sub.2n-2 (members with n=1,3,5,9 have been reported). Only titanates with layered structure, such as Na.sub.2 Ti.sub.4 O.sub.9 and Na.sub.4 Ti.sub.9 O.sub.20, exhibit ion exchange properties. The distinction between titanates and hydrous titanium, oxides is not very clear. Hydrous titanium oxides prepared in alkali metal form can, however, be considered as amorphous or semicrystalline/precrystalline forms of titanates. Hydrous titanium oxides are known to have ion exchange groups at least on their surfaces.
Although, as mentioned, titanates and hydrous titanium oxides are known to be efficient exchangers for radioactive strontium, they have not been used on an industrial scale yet. This is mainly because of the difficulties in preparing them in granular forms which should be resistant to high temperatures and radiation doses and suitable for packed bed use.
There are a number of known methods for preparing hydrous titanium oxides and titanates. The methods are summarized in the following:
1) Precipitation of hydrous titanium oxides from aqueous titanium solutions, especially TiCl.sub.4, with alkali solutions, especially NaOH, at room temperature (Abe, M., Wang, P., Chitrakar, R., and Tsuji, M. Adsorption and Desorption Behaviour of Heavy Metal Ions on Hydrated Titanium Dioxide, Analyst 114 (1989)435). PA1 2) Boiling of amorphous products from the precipitation of TiCl.sub.4 with NaOH or other solid hydrous titanium oxides, such as hydrous anatase, in concentrated NaOH solution (Heinonen, O. J. , Lehto, J., and Miettinen, J. K., Sorption of Strontium (II) and Radio Strontium Ions on Sodium Titanate, Radiochim. Acta 28 (1981)93). PA1 3) Hydrothermal treatment of amorphous products from the precipitation of TiCl.sub.4 with NaOH or other solid hydrous titanium oxides, such as hydrous anatase or a sol-gel product, in concentrated NaOH solution. PA1 4) Solid state synthesis of crystalline titanates, such as Na.sub.2 Ti.sub.6 O.sub.7, from solid titanium compounds, such as TiO.sub.2, and a sodium salt, such as Na.sub.2 CO.sub.3, at high temperatures, 700-1100.degree. C. (GE Patent No. 1,560,623 (1980)).
The precipitation method gives rise to amorphous non-granular products with rather low capacities. Since the process is rapid the products are usually non-homogeneous and non-reproducible.
To obtain more homogeneous products sol-gel methods have been used, in which better mixing of the reagents has been obtained by mixing titanium alkoxide with NaOH dissolved in alcohol. This results in the formation of a soluble titanate intermediate, which can be precipitated by adding water.
This treatment increases both the crystallinity and capacity of the product.
Rather high temperatures, 200-500.degree. C., and pressures, 20-400 bars, are needed for the hydrothermal treatment. The synthesis produces crystalline or semicrystalline titanates, such as Na.sub.4 Ti.sub.9 O.sub.20.xH.sub.2 O (Clearfield, A., and Lehto, J., Preparation, Structure and Ion Exchange Properties of Na.sub.4 Ti.sub.9 O.sub.20.xH.sub.2 O, J. Solid State Chem. 73 (1988)98).
A review of the patent literature shows that there are several patents on alkali metal titanates as such. Many of products are prepared by processes which use elevated and high temperatures in solid state synthesis (method 4 above) (U.S. Pat. No. 1,697,929 (1929), FI Patent Application No. 2665/72 (1972), GB Patent No. 1,560,623 (1980), DE Patent No. 619,568, DE Patent No. 497,626. U.S. Pat. No. 3,993,740 describes a method to produce fibrous potassium titanate with a hydrothermal method.
Preparation methods for titanates to be used as ion exchangers for strontium removal are described in WO Patent Application No. WO 83/03819, U.S. Pat. No. 4,161,513 (1979) and UK Patent 1,493,698. They are mainly based on method 2 above.
WO 83/03819 describes a method for manufacturing titanates from a titanium dioxide hydrate, which is suspended in water and alcohol. After heating , a base is added to the boiling mixture. By varying the reacting base it is possible to vary the selectivity of the product towards different metals. The titanates prepared have only moderate capacities for waste nuclides.
U.S. Pat. No. 4,161,513 describes a method for preparing ion exchanger titanates where TiCl.sub.4 is dissolved in alcohol or a ketone and the mixture is thereafter partially neutralized and chloride is removed after precipitation with a proper base. Finally titanate is precipitated with a base and water.
GB Patent No. 1,493,698 describes a method to prepare ion exchanger titanates from titanyl alkoxides. This method is a typical sol-gel method wherein a base, e.g. NaOH, in alcohol solution is mixed with titanium alkoxide to form a homogeneous soluble intermediate, which is precipitated with water. The composition of the product is NaTi.sub.2 O.sub.5 H.
The above-mentioned known processes are hampered by considerable disadvantages. Thus, WO 83/03819 fails to describe a method for preparing granular titanates which are suitable for column use, and both the methods given in U.S. Pat. No. 4,161,513 and GB Patent No. 1,493,698 use rather expensive raw materials and are complicated multistage processes. Furthermore, the capacities of the known ion exchangers are rather low.
In order to complete the survey of the related art, it should finally be mentioned that there are also patents describing methods to produce granular titanates in composite materials.
Thus, CZ Patent No. A.O. 273,369 describes a method to produce grains of composite materials containing various inorganic ion exchange materials, including sodium titanate, in polyacrylonitrile (PAN) binder. It is highly improbable that PAN as an organic polymer would withstand high irradiation doses generated in the columns especially if highly radioactive waste solution were treated with such an exchanger. U.S. Pat. No. 5,298,199 describes a method to incorporate sodium titanate inside the pores of zeolite. The performance of such an exchanger in strontium removal is rather poor compared to the sodium titanate of this invention. The distribution coefficient for this known product, is rather poor, viz. measured in 2.1M sodium ion solution only 1,958 mL/g at pH 11.1.